With appropriate electrical loading, photovoltaic solid state semiconductor devices, commonly known as solar cells, convert sunlight into electrical power by generating both a current and a voltage upon illumination. The current source in a solar cell is the charge carriers that are created by the absorption of photons. These photogenerated carriers are typically separated and collected by the use of PN or PIN junctions in semiconductor materials. The operational voltage of the device is limited by the dark current characteristics of the underlying PN or PIN junctions, among other limiting characteristics. Thus improving the power output performance of any solid state solar cell generally entails simultaneously maximizing absorption and carrier collection while minimizing dark diode current.
The solar electric conversion efficiency of conventional solar cells decreases as the cell thickness decreases due to incomplete absorption and the resulting decrease in short circuit current. However, the dark current can also decrease with decreasing cell thickness, particularly for photovoltaic devices operating in the radiative limit, resulting in an increase in open circuit voltage. It is desirable to leverage the inherent lower dark current of optically thin absorber layers. By incorporating light trapping structures, optically-thin solar cells can match or even exceed the performance of optically thick photovoltaic devices. Optically-thin photovoltaic devices also offer advantages compared to conventional structures in terms of increased robustness (including enhanced radiation tolerance) and reduced material costs. Photovoltaic modules employing thin-film technologies based upon cadmium telluride (CdTe), amorphous silicon, and copper indium gallium di-selenide (CIGS) materials can be manufactured at lower costs per unit area than crystalline silicon technologies. Thin-film cells use less semiconductor materials than crystalline silicon cells, and can, in some cases, be manufactured in a flexible format. It is desirable to increase the efficiency of thin-film solar cells and lower the cost of photovoltaic systems.